Hi, If this is an electric dryer, check the 4" vent going outside and make sure it is clean.
If this is a gas dryer, you caould have the coils on top the gas valve going bad.
Please let me know if I can assist you further.

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First checkup the coolant level in the radiator before you start the engine cold. Top off as necessary. If it's rally low you may need to "burp" the air from the cooling system.

When the radiator is full start the vehicle and watch the temp gauge. Once the gauge reaches the midpoint you should have plenty of heat. If there is still less than adequate heat raise the rpm slightly. If a lot of heat starts to come from the vents with the added RPMs than one of two things has happened.

1. The coolant level is still to low. The added rpm is enough to push coolant up to the heater core but low RPMs cause a loss of coolant flow and thus less heat.

2. The heater core is starting to plug up due to dirty old neglected coolant. The extra RPMs cause more flow which leads to more heat.

check to make sure your cooling fan isn't running constantly, also if your temp gauge says the engine is hot but the heat isn't you have a flow problem. maybe water pump or a clogged/ crimped hose. if your gauge isn't very hot then put some cardboard infront of the grill.

Check the thermostat. It's almost like a "float" that allows coolant to pass through a hose. If it gets stuck, you have heating issues. Since yours seems to fluctuate, I'd bet it's a thermostat about to go. Fortunately, it's easy to fix and inexpensive.

If thermostat has been changed and still not hot enough, I'd take a look at this link for my tip regarding poor heat. Remember to bleed system good and make sure you have heater on when re-filling/bleeding the system. Quite often at idle, it's poor, and when RPM's are increased the heater feels hotter in this case.( http://www.fixya.com/cars/r3698796-unclog_heater_core_rather_replace )

does the heater core get hot, check the coolant hoses going to the core to see if both are hot, if they are U have a air temperature control blend door problem, most likely the actuator for the door has failed.